Police are investigating the death of an experienced diver yesterday during deep water training at a scuba diving centre that has been the scene of numerous fatalities.

The victim, who has not been named, encountered difficulties during a programme to improve "deep diving" skills at the National Diving Centre at Stoney Cove, near Hinckley in Leicestershire.

As it became clear that he was in danger, four divers from the same party put themselves at serious risk by bringing their stricken colleague to the surface at speed. He was later pronounced dead at Leicester Royal Infirmary.

The four are believed to have suffered decompression sickness, also known as the bends, and were airlifted to hospital by an RAF Sea King helicopter from RAF Kinross. Yesterday, as the police began trying to find out what went wrong and health and safety officials launched their own inquiry, hospital officials from the Murrayfield Hospital in Cheshire said the four divers were out of danger and slowly recovering. The men were treated in a recompression unit to rid them of the potentially lethal nitrogen bubbles that form in the body as a result of the bends.

A centre spokesman said the group were from a Derbyshire diving school and were not under the instruction of Stoney Cove. "Very soon after the start of the dive one diver came to the surface, raising the alarm that another was in difficulty. The Stoney Cove rescue team responded instantly, and almost immediately other divers in the group brought the distressed diver to the surface."

John Bantin, technical editor of Diver magazine, said their bravery should be applauded. "They risked their own lives and made a brave decision to try and save him," he said.

The National Diving Centre is Britain's biggest inland diving centre. The 13-acre site was a quarry until 1958 when spring water was allowed to flood the quarry workings. In the 1960s and 70s it was used to train commercial divers before they went out to the North Sea.

It is immensely popular with divers from all over the country who come to dive in one of its four pools, one of which includes the Stanegarth, the UK's largest inland shipwreck.

The facility offers depths of seven, 22 and 36 metres, depending on the diver's experience. Work recently started on a new dive centre building which, when finished, will contain a medical centre.

Mr Bantin said: "It is an inherently easy place to go diving because the water is calm and unaffected by bad weather. There are no currents. But people are still putting themselves under pressure and the inherent dangers of diving are still there."

At least one diver has drowned each year since the centre opened in 1978. Three years ago, the centre was fined £7,500 and ordered to pay more than £40,000 in costs following the death of an amateur diver. An investigation revealed that Paul Gallacher, 33, had not completed enough training when he was allowed to take part in a dive that proved fatal. In a prosecution brought by the Health and Safety Executive, the centre admitted it had failed to ensure divers were not exposed to risks.

Two other divers died last year despite exploring the depths using the "buddy" system, where divers keep watch over each other.

Officials, however, insist that the centre has a good record for safety and say the death rate over the past 28 years should be put into perspective.

For all the controversy, Stoney Cove remains massively popular. Entry queues often begin at 5am. At 2pm yesterday, four hours after the accident, the centre was still permitting dives to take place. The most frequent cause of diving injuries or fatalities comes from decompression sickness which occurs when a diver comes to the surface too quickly, causing the nitrogen which has dissolved in their body to expand.